The presence of distortion in high frequency amplifiers or signal processors is difficult to avoid and can cause substantial degradation in overall system performance. This is particularly true when travelling wave tube (TWT) amplifiers and high powered radio frequency (HPRF) amplifiers are utilized for amplification and/or processing of coherent communication signals.
For example, non-linearities in the transfer characteristics of TWT and/or HPRF amplifiers can severely distort phase modulated waveforms. Phase-shift keying (PSK) and quadrature amplitude modulation (QAM) are well known phase modulation techniques. Variations in phase and/or amplitude arising from non-linear amplifier transfer characteristic have the affect of decreasing the effective signal to noise ratio in the amplified signal, whether in a transmitter or receiver or both, i.e., the data error rate increases.
Various signal correction methods have been tried in the past. For example, FIG. 1 shows a schematic representation of prior art adaptive, digital, pre-distortion linearized amplifier system 10 employing non-linear radio frequency amplifier 12. This prior art system is described in greater detail by A. A. M. Saleh et al., in "Adaptive Linearization of Power Amplifiers in Digital Radio", Bell System Technical Journal, Vol. 62, Apr. 1983, pages 1019-1033, so only a brief explanation is provided here.
Digital input data 14 embodying the information desired to be modulated onto a carrier, amplified and transmitted is provided to memory look-up encoder 16 and linearizing processor 18. Lookup encoder 16 provides a transformation between the incoming digital data stream and the bit stream later used, via random access memory (RAM) 20 and D/A converter 22, to provide in-phase signal I and quadrature signal Q to quadrature modulator 24 which in turn generates modulated RF signal 26 to power amplifier 12 which provides amplified modulated RF signal 27 to directional coupler (DC) or splitter 28 and modulated RF signal 30 to an antenna. The RF carrier is supplied by local oscillator 32. In FIG. 1 (and FlG. 6), heavy solid lines indicate digital signal paths, light solid lines indicate analog signal paths and dashed lines indicate RF signal paths.
Portion 34 of amplified modulated RF signal 27 is obtained from splitter 28 from whence it flows to quadrature demodulator 36 where it is demodulated and the analog output fed to A/D converter 38 to provide a bit stream analogous to that which was supplied to D/A converter 22 but now modified according to the distortion introduced by amplifier 12. This feed back digital signal is compared in linearizing processor 18 with incoming bit stream 14 and used to modify the bits stored in RAM 20 which are sent to D/A converter 22 to drive modulator 26 and amplifier 12 so that the difference between fed-back digital signal 39 and incoming digital signal 14 is minimized. In summary, incoming signal 14 is pre-distorted by providing in RAM 20 additional bits corresponding to the distortion introduced by amplifier 12 so that the resulting information contained on modulated RF output 30 is substantially un-distorted, i.e., the pre-distortion added to the signal to be amplified is such as to substantially compensate for the distortion introduced by amplifier 12. An advantage of the system of FIG. 1 is that the exact form of the distortion introduced by amplifier 12 need not be known in advance since the system is adaptive.
While the system illustrated in FIG. 1 provides improved system performance as compared to amplifier 12 alone, it is complex and expensive to implement and, depending on the data rate and RF carrier frequency, may consume more power than can be conveniently provided, particularly in satellite applications. A further difficulty is that the arrangement of FIG. 1 is inherently slow, being limited by, among other things, the RAM read/write times. It is extremely difficult if not impossible with such an approach to provide real time, error corrected, signal processing and/or amplifying systems that operate with signalling rates approaching 100 MegaHertz or higher.
Accordingly, there is a continuing need for improved electronic signal processing apparatus, as for example, RF amplifying systems operating typically in the range 5-10 GigaHertz or higher that provide reduced distortion in a less complex, more economical and/or more power efficient manner and which can operate at high data rates. This is especially important in connection with satellite based, coherent (i.e., phase synchronized) RF systems where weight and power consumption are especially critical and high data rates are desired.